Well there are several open source libraries available for C++:
1. Xerces XML Parser ( most complete, most popular, but huge and higher learning curve)
2. TinyXML ( small, simple, and easy to learn but doent support Schema, DTD etc...)
3. MSXML

You can also search for XML Parser in www.sourceforge.net.

There's also something called XMLLite from Microsoft. Read about that in MSDN but haven't tried it yet.